Closed-loop sputtering systems have been employed to deposit thin films of a material, such as a metal or metal oxide, in a controlled manner onto a substrate surface. Typically such closed-loop sputtering systems operate through the creation of a plasma of ionized gas between target (cathode) and anode electrodes whereby in operation a molecular film of the target electrode is deposited and condensed in a thin-film form onto a substrate between the electrodes.
In DC sputtering systems of the types now in use, control of the plasma in a sputtering work chamber containing the electrodes is accomplished by monitoring the pressure of the gas within the work chamber through the use of a transducer as a pressure-sensing device to provide an error-output signal in reference to a preset reference signal with the error signal obtained by a comparison of the electrical signal from the pressure in the chamber to the reference signal to provide for an error-control signal to a piezoelectric valve, which valve in turn proportionally controls the flow of the ionizable gas to the plasma. The error signal from the control sets the valve opening so as to maintain the pressure of the work chamber at a desired level.
In the pressure-controlled system there are typically pressure gradients in the work chamber which are functions of the geometry of the chamber. As parts move in the field of flow this gradient is typically altered. In addition, the pressure transducer is usually remote from the plasma and thus measures the pressure with a variable error depending on the flow and geometry of the chamber. Thus, the pressure control is variable due to these factors as well as the time delay in said plasma disturbances in pressure output and the errors in changing from transducer to transducer, all of which are overcome through the use of my invention.
Present DC closed-loop sputtering systems are directed to maintaining the lowest-possible pressure level within the work chamber, that is at a base pressure level, for example, of about 1 micron, which will permit the continuous nonextinguishing operation of the plasma. However, such systems have disadvantages in that the pressure level of the work chamber must be maintained at a level sometimes 3 to 5 times or more higher than the base pressure level in order to compensate for pressure fluctuations in the work chamber during operation of the plasma and for a response time and inaccuracies in the pressure transducer and in the operation of the piezoelectric valve in the system. Thus, the pressure level is often much higher than the base pressure to compensate for such variations and to prevent the plasma from extinguishing by having the pressure fall below the base pressure. In order to obtain thin films of uniform thickness and structure it is most desirable to maintain a high degree of control over the operation of the plasma in the work chamber and to operate continuously at a pressure just above the base pressure. Therefore a closed-loop sputtering system to provide for a better control of the plasma and at a lower level than possible by present systems would be most desirable in that the sputtering system would produce a higher-quality controlled and more uniform film.